I havent been doing this very long, but I have been putting my listing link at the bottom of craigslist listings and using the cross sell application (free) you can get on ebay. I am sure there is a lot more- but it has helped me the last two months.
Ask your bidders to sign up for your newsletter or add you to their fav sellers list. Keep a computer file or index cards on who buys what from you, with their contact info. Send emails or snailmail postcards to them at least once a year to remind them they bought from you before and why they should again.